The invention is related to the field of optics, and in particular to forming optical structures, such as isolators, using magnetic materials.
An isolator is a device that allows polarized light to pass through in one direction, but not in the opposite direction (like a one-way valve). These are useful in photonic applications. An isolator is placed at the outlet of a laser, allowing the light to leave the laser, but not to reenter it, which would make the laser unstable. An isolator typically consists of a pair of polarizers, with their principal axes oriented at 45°, placed around a magneto-optical crystal. The crystal rotates the plane of polarization of light passing through it: this property is called Faraday rotation. The crystal is thick enough that it rotates the plane of polarization of the light by exactly 45°, and the light can therefore pass through the second polarizer. However, if light is propagating towards the laser, it is blocked by the first polarizer and cannot pass.
For optical communications, which operate at a laser wavelength of 1550 nm, isolators are made from a bismuth-substituted yttrium iron garnet material (Bi—YIG) which has a high Faraday rotation of up to about 6° per micron, depending on the Bi content. This means that the isolator crystal needs to be 45°/6=7.5 microns long, or longer for a material with lower Faraday rotation. Isolators are made as discrete devices by growing crystals of Bi—YIG, for example by liquid-phase epitaxy on other garnet substrates, then cutting them into the correct shape and mounting them between polarizers.
Recently, however, there has been interest in making integrated photonic devices in which the isolator is formed as a component on a chip, integrated with the other components (lasers, waveguides, etc.) For an integrated optical device, it is important to note that there are various designs of isolators other than that shown in the figure above, for instance devices based on Mach-Zehnder interferometers. These other designs have the advantage that they do not require separate polarizers and analyzers, making them more suitable for integration. Additionally, they can work successfully using magneto-optical materials with relatively modest values of Faraday rotation, e.g. less than 0.1°/micron.
For making an integrated isolator, the following properties are the most important: (1) the material must have a Faraday rotation, but it can be modest, in the range of ˜0.01°/micron or greater; (2) the material must have a high degree of transparency at the relevant wavelength (1550 nm), and (3) the material must be compatible with a substrate such as GaAs or Si, meaning that it can be grown as a good quality crystal and subsequently processed on a GaAs or Si substrate.
Bi—YIG satisfies the first two criteria but not the third. It is hard to grow as a single crystal film on a substrate other than garnet, so if it is grown on Si or GaAs it does not have the required magneto-optical properties. There is therefore interest in finding alternative materials with high Faraday rotation that can be integrated on Si or GaAs substrates.